1. Field
Example embodiments relate to an optical recording medium, and for example, to a method and apparatus for converging voltage levels of a plurality of input signals of an optical recording medium to a reference voltage.
2. Description of Related Art
For discs, e.g., compact discs (CDs), and magneto-optical discs that may be recorded on an optical recording medium, a recording area is generally divided according to a specific basic recording unit in order to perform a recording operation in a specific unit. An optical pick-up unit may need to exactly find a physically divided area that is the basic recording unit at a high speed without causing an error in order to write or read data onto or from the physically divided area. The optical recording medium changes a DC voltage level of an input signal, while the optical pick-up unit operates, to a reference voltage for the inside of the optical recording medium. A high pass filter (HPF) is generally used to change the DC voltage level of the signal to the reference voltage.
FIG. 1 is a circuit diagram of a conventional HPF 100. Referring to FIG. 1, the conventional HPF 100 uses a resistor R and a capacitor C. A reference voltage Vref may be applied to one of the resistor R. If a time function, which is a signal, e.g., x(t), is input to an input end INPUT of the HPF 100 and another time function y(t) is output from an output end OUTPUT of the HPF 100 in response to x(t), a transfer function H(s) of the HPF 100 is expressed by Equation 1 as is well known to those of ordinary skill in the art.
                              H          ⁡                      (            s            )                          =                                            Y              ⁡                              (                s                )                                                    X              ⁡                              (                s                )                                              =                      sRC                          1              +              sRC                                                          (        1        )            
A cutoff frequency fc indicated in Equation 1 is expressed by Equation 2 as is also well known to those of ordinary skill in the art.
                              f          c                =                  1                      2            ⁢            π            ⁢                                                  ⁢            RC                                              (        2        )            
FIG. 2 is an example graph representing the transfer function H(s) of the HPF 100 illustrated in FIG. 1. Referring to FIG. 2, the graph showing the transfer function H(s) indicates that the input signal is not transmitted in an area of frequencies lower than the cutoff frequency fc, whereas the input signal is transmitted from the output end OUTPUT of the HPF 100 without loss in an area of frequencies higher than the cutoff frequency fc.
If a DC voltage level of the input signal is changed to a reference voltage of an optical recording medium, the DC voltage level of the input signal does not quickly change due to response speed characteristics of the fixed cutoff frequency fc of the conventional HPF 100. In order to address this problem, a method of changing the cutoff frequency fc of the conventional HPF 100 to be higher by designing a variable cutoff frequency fc to have faster response speed characteristics has been suggested. However, the method uses several cutoff frequencies fc, which causes an increase in chip size. Furthermore, the higher cutoff frequency fc reduces a resistance of the resistor R of the conventional HPF 100, which makes driving a current more difficult. The size of the capacitor C of the HPF 100 needs to be reduced in order to increase the resistance of the resistor R of the HPF 100.